Traumatic brain injury (TBI) is a signature injury of OIF/OEF Veterans. More than 360,000 armed service members sustained TBI during combat and training from 2000 to 2016. There is currently no diagnostic biological marker for TBI nor can current diagnostic tools identify individuals at greatest risk for chronic neurological and subsequent functional impairments after TBI. Neuronally-derived exosomes (NDEs) obtained from peripheral blood may be a powerful tool to develop accessible CNS-based biomarkers associated with neuronal dysfunction, particularly in relation to long-term brain injury and neurodegeneration. Our recently published work demonstrates that neuropathological proteins (e.g betaamyloid, A, and tau) within NDEs can predict conversion from mild cognitive impairment to Alzheimer's Disease (AD) while plasma levels do not. Studies of NDEs are now being tested as companion biomarkers in AD clinical trials to help reduce screen fail rates and increased enrollment. We have recently found that cytoskeletal and synaptic proteins are also abnormal in deployment-related TBI patients >3 mo after TBI. Specifically, both A and neurogranin are altered in plasma NDEs from participants who experienced a deployment-related TBI. Taken together, our data support the hypothesis that plasma NDEs may be a powerful tool to identify accessible and CNS-specific protein biomarkers for TBI. NDEs are also enriched for short length ?micro? RNA (miRNA) cargo. Each miRNA can regulate protein expression from hundreds of target messenger RNAs, providing an efficient mechanism to exert genome-wide regulation and simultaneously affect several cellular pathways. miRNAs confer tissue specificity and have recently emerged as potential biomarkers and therapeutic targets for neurodegeneration. Hence, identification of CNS-specific miRNAs associated with NDEs may provide a window to the pathogenic processes in chronic TBI for future intervention. We hypothesize that proteins related to neurodegeneration within NDEs, such as A, as well as miRNAs associated with NDEs have the potential to be biomarkers of TBI and associated symptoms. To test this hypothesis we will leverage our prospective longitudinal study of combat deployment effects in >1200 Marines, of which 176 experienced mild/moderate deployment-related TBI. This study collected physical and mental health, neurocognitive performance and blood samples 1 mo before and 4-6 mo after a combat deployment to Afghanistan. We will use 150 TBI samples with 150 samples of matched controls with no TBI history to complete 2 aims. Aim 1 will examine utility of cytoskeletal and neuronal proteins in NDEs to identify TBI in addition to persistence of post-concussive symptoms and cognitive decline. Candidate neurodegenerative proteins include tau, A, neurogranin, neurofilament light chain and calpain-cleaved ?II-spectrin N-terminal fragment. Studies will also leverage these prospective samples to examine if change in NDE proteins from pre-and post injury reflects symptom change, providing potential causal inferences for these proteins in the pathogenic process of TBI. Aim 2 will use an unbiased discovery approach to identify novel miRNAs associated with TBI and associated symptoms and neurocognitive decline. Small RNA sequencing will be conducted on exosomal RNA. miRNA associated with TBI will be validated using RT-qPCR and replicated in an independent sample set. Top candidates will be examined for changes pre and post injury to understand the contribution of these markers to the pathogenic process after injury. This study has strong potential to provide accessible, quantitative biomarkers for TBI and associated symptoms, as well as identify potential functional targets for intervention. ! !